Local probe microscopy is capable of high-resolution non-destructive measurement of one or more characteristics of a specimen's surface. A typical AFM operated in contact mode includes a probe comprising a cantilever with a probe tip mounted near the free end of the cantilever. The AFM probe is mounted on a stage having actuators which provide x, y and z motion of the probe tip relative to a sample stage. An optical lever system (laser reflection from the back of the cantilever onto a segmented photodiode) or an alternative position sensitive detector is used to monitor deflection of the free end of the cantilever which arises from changes in the interaction between the probe tip and the surface of a sample on the sample stage. In constant force scanning, as the probe is moved in the x, y plane so as to scan across the surface of the sample, the height of the probe above the sample, i.e. in the z direction, is adjusted to maintain a fixed deflection of the cantilever. These z direction adjustments maintain a constant interaction force between the probe tip and the sample surface for a particular x, y location and are representative, for example, of the topography of the sample surface. With a conventional AFM of the type described above lateral and vertical resolutions of better than 2 nm and 0.1 nm respectively are achievable. Under the correct conditions, lateral resolution down to the atomic level can be achieved. However, the operation and performance of the microscope is dependent upon a knowledge of the spatial relationship between the sample and the probe. Inevitably this spatial relationship is affected, and thus the resolution is limited, by amongst other things the operating environment and in particular mechanical and acoustic noise which generate an uncertainty in the position of the probe relative to the specimen.
Various improvements have been proposed in recent years to address the problem of noise with respect to local probe microscopy. In U.S. Pat. No. 6,196,061 and U.S. Pat. No. 6,279,389, for example, an atomic force microscope (AFM) is described which employs two separate probes: one a measurement probe, the other a reference probe mounted on opposite corners of a single cantilever. The reference probe is flattened (made dull) so as to average the specimen topography as the flattened probe is scanned over the specimen. Additionally, the microscope includes a structure which mechanically links the specimen platform to the structure supporting the local probe. Deflection of the cantilever is measured optically from a point on the back of the cantilever midway between the two probes and thus generates a signal with a reduced noise component that provides differential deflection measurements with respect to the surface area adjacent the measurement probe and thus is not independent of the scanning measurements. However, the AFM is restricted to differential deflection data; no absolute deflection measurements of the measurement probe can be produced using this microscope. These two patents also describe an alternative probe consisting of multiple cantilevers arranged parallel to one another with interdigitated fingers along each side edge. The interdigitated fingers act as diffraction gratings again providing differential deflection data.
In US 2003/0020500 and US 2005/0040836 a probe having two or more probe tips each individually mounted on a respective cantilever but with common position control is described. The individual probe tips are arranged so that one tip, a distance sensor, is positioned lower, i.e. closer to the sample stage, than the remaining probe tips. Unlike U.S. Pat. No. 6,196,061 and U.S. Pat. No. 6,279,389, with this version of a probe having multiple cantilevers, the deflection of each cantilever is separately monitored using, for example, a conventional optical lever technique. The distance sensor provides a reference height measurement from the sample surface and is used to maintain the distance of the remaining probe tips in the z direction from the surface of the sample. This permits the surface of the sample to be scanned using the probe tips at well defined distances above the surface of the sample. However, as a result of the common position control, the height reference measurement of the distance sensor is not independent of the scanning measurements.
Semiconductor device fabrication involves highly complex techniques for constructing integrated circuits using semiconductor materials. Due to the large scale of circuit integration and the decreasing size of semiconductor devices, the semiconductor fabrication process is prone to processing defects. Current testing procedures, involving separate inspection and review processes, are therefore critical to maintain quality control. Since the testing procedures are an integral and significant part of the fabrication process, the semiconductor industry is constantly seeking for more accurate and efficient monitoring procedures.
The inspection process involves an initial scan of the surface of the semiconductor wafer by a high-speed, relatively low-resolution inspection tool. A defect map is then produced showing suspected locations on the wafer having a high probability of a defect. When a review of affected wafers is warranted, either the optics of the inspection apparatus must be changed to a higher (review) resolution or the wafers are transferred to a different tool altogether. To perform the review, the defect map is fed to the review apparatus and then individual suspected defect sites are more closely scrutinised by means of a relatively high-resolution image. That image is analysed, for example involving the use of pattern recognition techniques in the case of automated defect classification (ADC), to determine the nature of the defect i.e. to classify the defect in order to discriminate between, for example, a defective pattern, damage caused by a foreign particle or a surface scratch.
Optical methods such as dark and bright field microscopy have been used in wafer defect review and classification processes. However, such optical devices are limited in their ability to analyze and accurately identify defect types on a semiconductor wafer. This is especially the case as the thrust of future development in the semiconductor device field is to ever higher densities of integrated circuits, involving sub-micron and sub 100 nm device features, as well as structures with increased aspect ratios. As a result, due to its greater resolution, scanning electron microscopy or “SEM” is favoured in defect review and classification of features on the wafer. However, scanning electron microscopy has the disadvantage that not only must the wafers be removed from the production line; they have to be transferred to an evacuated enclosure for the purposes of electron microscopy. This can add considerable time to the inspection and review procedure, which increased time is undesirable. Moreover, scanning electron microscopy can result in damage to the wafer surface via beam damage and/or contamination.
Local probe microscopy techniques, such as those described above, are capable of providing high resolution information on surface features of a sample. However, due to the length of time required to produce a surface scan using conventional scanning probe microscopy apparatus, the application of such microscopy techniques is limited in large-scale wafer fabrication. Moreover, as with all local probe techniques, the resolution and capabilities of such local probe microscopy apparatus are directly affected by environmental mechanical vibrations, especially where a local probe is mounted in a conventional inspection/review tool. The effect of environmental mechanical vibrations is increased when using a long mechanical path between the probe and sample as is required for large samples such as wafers used in semiconductor device fabrication. Additionally, the need to move and reposition the sample, e.g. a wafer, introduces mechanical vibrations which must settle before a measurement can be collected and this reduces throughput.
Scanning probe microscopes which employ interferometers for vibration and noise cancellation have been described in U.S. Pat. No. 6,178,813 and U.S. Pat. No. 7,249,002. In each of these documents in addition to the usual scanning probe system an interferometer is used to monitor the separation between a point on the sample surface and a fixed point. Measured variations in this separation are used to compensate for noise in the measurements produced by the scanning probe. However, microscopes employing these noise compensation techniques require two separate measurement systems and are thus costly. More importantly the interferometers are only capable of measuring displacements, representative of noise amplitudes, of less than half a wavelength.